docs: def get_aids_by_tag(self):
          """
          :returns: dict tag -> asset ordinals
          """
          aids_by_tag = general.AccumDict(accum=set())
          for aid, ass in enumerate(self):
              for tagname in self.tagnames:
                  tag = self.tagcol.get_tag(tagname, ass[tagname])
                  aids_by_tag[tag].add(aid)
          return aids_by_tag